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The active site of hydroxynitrile lyase from Prunus amygdalus: modeling studies provide new insights into the mechanism of cyanogenesis.

ABSTRACT: The FAD-dependent hydroxynitrile lyase from almond (Prunus amygdalus, PaHNL) catalyzes the cleavage of R-mandelonitrile into benzaldehyde and hydrocyanic acid. Catalysis of the reverse reaction-the enantiospecific formation of alpha-hydroxynitriles--is now widely utilized in organic syntheses as one of the few industrially relevant examples of enzyme-mediated C-C bond formation. Starting from the recently determined X-ray crystal structure, systematic docking calculations with the natural substrate were used to locate the active site of the enzyme and to identify amino acid residues involved in substrate binding and catalysis. Analysis of the modeled substrate complexes supports an enzymatic mechanism that includes the flavin cofactor as a mere "spectator" of the reaction and relies on general acid/base catalysis by the conserved His-497. Stabilization of the negative charge of the cyanide ion is accomplished by a pronounced positive electrostatic potential at the binding site. PaHNL activity requires the FAD cofactor to be bound in its oxidized form, and calculations of the pKa of enzyme-bound HCN showed that the observed inactivation upon cofactor reduction is largely caused by the reversal of the electrostatic potential within the active site. The suggested mechanism closely resembles the one proposed for the FAD-independent, and structurally unrelated HNL from Hevea brasiliensis. Although the actual amino acid residues involved in the catalytic cycle are completely different in the two enzymes, a common motif for the mechanism of cyanogenesis (general acid/base catalysis plus electrostatic stabilization of the cyanide ion) becomes evident.

SUBMITTER: Dreveny I

PROVIDER: S-EPMC2373431 | biostudies-literature | 2002 Feb

REPOSITORIES: biostudies-literature

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The active site of hydroxynitrile lyase from Prunus amygdalus: modeling studies provide new insights into the mechanism of cyanogenesis.

Dreveny Ingrid I Kratky Christoph C Gruber Karl K

Protein science : a publication of the Protein Society 20020201 2

The FAD-dependent hydroxynitrile lyase from almond (Prunus amygdalus, PaHNL) catalyzes the cleavage of R-mandelonitrile into benzaldehyde and hydrocyanic acid. Catalysis of the reverse reaction-the enantiospecific formation of alpha-hydroxynitriles--is now widely utilized in organic syntheses as one of the few industrially relevant examples of enzyme-mediated C-C bond formation. Starting from the recently determined X-ray crystal structure, systematic docking calculations with the natural substr ...[more]

PMID: 11790839

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Project description:Bacterial populations present in Hg-rich environments have evolved biological mechanisms to detoxify methylmercury and other organometallic mercury compounds. The most common resistance mechanism relies on the H(+)-assisted cleavage of the Hg-C bond of methylmercury by the organomercurial lyase MerB. Although the initial reaction steps which lead to the loss of methane from methylmercury have already been studied experimentally and computationally, the reaction steps leading to the removal of Hg(2+) from MerB and regeneration of the active site for a new round of catalysis have not yet been elucidated. In this paper, we have studied the final steps of the reaction catalyzed by MerB through quantum chemical computations at the combined MP2/CBS//B3PW91/6-31G(d) level of theory. While conceptually simple, these reaction steps occur in a complex potential energy surface where several distinct pathways are accessible and may operate concurrently. The only pathway which clearly emerges as forbidden in our analysis is the one arising from the sequential addition of two thiolates to the metal atom, due to the accumulation of negative charges in the active site. The addition of two thiols, in contrast, leads to two feasible mechanistic possibilities. The most straightforward pathway proceeds through proton transfer from the attacking thiol to Cys159 , leading to its removal from the mercury coordination sphere, followed by a slower attack of a second thiol, which removes Cys96. The other pathway involves Asp99 in an accessory role similar to the one observed earlier for the initial stages of the reaction and affords a lower activation enthalpy, around 14 kcal mol(-1), determined solely by the cysteine removal step rather than by the thiol ligation step. Addition of one thiolate to the intermediates arising from either thiol attack occurs without a barrier and produces an intermediate bound to one active site cysteine and from which Hg(SCH3)2 may be removed only after protonation by solvent-provided H3O(+). Thiolate addition to the active site (prior to any attack by thiols) leads to pathways where the removal of the first cysteine becomes the rate-determining step, irrespective of whether Cys159 or Cys96 leaves first. Comparisons with the recently computed mechanism of the related enzyme MerA further underline the important role of Asp99 in the energetics of the MerB reaction. Kinetic simulation of the mechanism derived from our computations strongly suggests that in vivo the thiolate-only pathway is operative, and the Asp-assisted pathway (as well as the conversion of intermediates of the thiolate pathway into intermediates of the Cys-assisted pathway) is prevented by steric factors absent from our model and related to the precise geometry of the organomercurial binding-pocket.

Dataset Information

The active site of hydroxynitrile lyase from Prunus amygdalus: modeling studies provide new insights into the mechanism of cyanogenesis.

Publications

The active site of hydroxynitrile lyase from Prunus amygdalus: modeling studies provide new insights into the mechanism of cyanogenesis.

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